As a wide-bandgap semiconductor, GaN is attractive for next-generation high-efficiency power converter applications. The capabilities of GaN-based HEMTs to date have been limited by the self-heating effect, which has been well documented in the literature. See R. Gaska, A. Osinsky, J. W. Yang, and M. S. Shur, “Self-Heating in High-Power AlGaN—GaN HFETs,” IEEE Electron Dev. Lett. 19, pp. 89-91 (1998); and H. I. Fujishiro, N. Mikami, and M. Hatakenaka, “Monte Carlo study of self-heating effect in GaN/AlGaN HEMTs on sapphire, SIC, and Si substrates,” Phys. Stat. Sol. (c) 2, no. 7, pp. 2696-2699 (2005).
Diamond integration has been proposed as a technique to reduce self-heating, but backside approaches using diamond substrates are limited by sample size, availability, and coefficient of thermal expansion (CTE) mismatch. See M. Alomari, A. Dussaigne, D. Martin, N. Grandjean, C. Gaquiere, and E. Kohn, “AlGaN/GaN HEMT on (111) single crystalline diamond,” Electron. Letters, vol. 46, 2010, pp. 299-301; and K. D. Chabak, J. K. Gillespie, V. Miller, A. Crespo, J. Roussos, M. Trejo, D. E. Walker, Jr., G. D. Via, G. H. Jessen, J. Wasserbauer, F. Faili, D. I. Bahić, D. Francis, and F. Ejeckam, “Full-wafer characterization of AlGaN/GaN HEMTs on free-standing CVD diamond substrates,” IEEE Electr. Dev. Lett., vol. 31, no. 2, pp. 99, 2010. A scalable topside process, however, is enabled by the unique properties of nanocrystalline diamond (NCD) thin films, including high thermal conductivity, small grain size, and optical transparency. See J. E. Butler and A. V. Sumant, “The CVD of nanodiamond materials,” Chemical Vapor Deposition, 14, 145 (2008).
A significant limitation to topside NCD approaches has been the lack of thermal stability of the Schottky gate. See M. Seelman-Eggebert, P. Meisen, F. Schaudel, P. Koidl, A. Vescan, and H. Leier, “Heat-spreading diamond films for GaN-based high power transistors,” Diamond and Relat. Mater. 10 (2001), pp. 744-749; and M. Alomari, M. Dipalo, S. Rossi, M. A. Diforte-Poisson, S. Delage, J. F. Carlin, N. Grandiean, C. Gaquiere, L. Toth, B. Pecz, and E. Kohn, “Diamond overgrown InAlN/GaN HEMTs,” Diamond & Relat. Mater. 20, pp. 604-608 (2011).
We have previously reported improved electrical performance and a 20% reduction in channel temperature with a “gate after diamond” process, which enables large-area, high thermal conductivity top-side diamond without damaging the Schottky gate. See M. J. Tadjer, T. J. Anderson, K. D. Hobart, T. I. Feygelson, J. D. Caldwell, C. R. Eddy, Jr., F. J. Kub, J. E. Butler, B. B. Pate, and J. Melngailis, “Reduced self-heating in AlGaN/GaN HEMTs using nanocrystalline diamond heat-spreading films,” IEEE Electr. Dev. Lett. Vol. 33, no. 1 (2012) pp. 23-25.
An additional approach is to investigate thermally stable materials that can be used as the gate contact. While a metal Schottky contact is traditionally used as the gate material, the key requirement for a depletion-mode HEMT gate contact is low reverse leakage so that negative potential can be applied to turn off the device. Therefore any rectifying junction can serve this purpose.
NCD electrodes have been shown to form a rectifying contact to SiC. See M. J. Tadjer, K. D. Hobart, J. D. Caldwell, J. E. Butler, K. X. Liu, C. R. Eddy, Jr, D. K. Gaskill, K. K. Lew, B. L. VanMil, R. L. Myers-Ward, M. G. Ancona, F. J. Kub, and T. I. Feygelson, “Nanocrystalline diamond films as UV-semitransparent Schottky contacts to 4H—SiC,” Appl. Phys. Lett. 91, 163508 (2007); and M. J. Tadjer, T. I. Feygelson, K. D. Hobart, J. D. Caldwell, T. J. Anderson, J. E. Butler, C. R. Eddy, Jr., D. K. Gaskill, K. K. Lew, B. L. VanMil, R. L. Myers-Ward, F. J. Kub, G. Sollenberger, and L. Brillson, “On the high curvature coefficient rectifying behavior of nanocrystalline diamond heterojunctions to 4H—SiC,” Appl. Phys. Lett. 97, 193510 (2010).
Based on this model, it is expected that NCD will form a rectifying contact to AlGaN and wider-bandgap ternary nitrides. While indium tin oxide (no) has been demonstrated as a transparent gate contact, NCD films offer the additional advantages of high thermal conductivity and chemical and thermal stability. See Y. Pei, K. J. Vampola, Z. Chen, R. Chu, S. P. DenBaars, and U. K. Mishra, “AlGaN/GaN HEMT With a Transparent Gate Electrode,” IEEE Electron Device Lett. 30, pp. 439-441 (2009).